1. Field of the Invention
The present invention relates to systems for optical measurement of strain during mechanical testing. In particular, the present invention relates to an improved optical grid analyzer for determining strain in sheet metal during or following a forming operation.
2. Description of the Prior Art
Discussions of optical measurement of strain or displacement during mechanical testing have appeared in the literature for over twenty years. Although these optical measurement techniques which have been described have often been complex and expensive, the inherent advantages of optical measurement have made these techniques justifiable in many cases.
A principal benefit of optical strain measurement is that it can be noncontacting. There are no stress raisers caused by contact points or knives. Frail specimens are not affected by the spring rate, dampening, or resonance of an attached extensometer. Furthermore, optical instrumentation can be external to the environment of the test (e.g. high temperatures, salt sprays, vacuum). This is advantageous to both the test specimen and the extensometry.
Optical strain measurement can be easier to use and more productive than conventional extensometry. It contributes to a higher test throughput because it does not require attachment to each test specimen before testing begins and removal from the test specimen after testing is completed. It also is not physically subjected to the potential of violent failure of the test specimen which can occur at the conclusion of a test.
A significant advantage of some optical strain measurement techniques is that principal strains can be measured regardless of their orientation to the extensometry. This is often very important when measurements are taken in regions with strain gradients or when the orientation of the principal strain is unknown or changing.
Circle grid analysis is an optical measurement technique for measuring the principal strains in sheet metal. A pattern of circles is photogridded or electrochemically etched onto the sheet metal prior to deformation. If the strain gradient is small with respect to the size of the circles, the circles will generally become elliptical during deformation of the sheet metal. The ratios of the major and minor diameters (or radii) of the deformed circle to the original circle diameter (or radius) are used to calculate the principal strains. The measurements are typically made with an optical comparator or a machinist's microscope.
The greatest use of circle grid analysis is in the generation of forming limit diagrams, which are plots of the limiting major principal strain as a function of the minor principal strain. Forming limit diagrams can be used for material evaluation, lubrication evaluation, and during die tryout.
The growth of the use of circle grid analysis has been hampered by several factors. First, measuring the deformed circles is a very tedious and time-consuming task. This results in delays in obtaining data, and in unhappy, under-utilized technicians. Second, the subjective nature of most of the circle measurement techniques results in inaccuracy in the data obtained by those techniques.
In 1979, Robert Ayres et al addressed these limitations in a paper describing a machine vision system for circle grid analysis. Robert A. Ayres, Earl G. Brewer, and Steven W. Holland, "Grid Circle Analyzer--Computer Aided Measurement of Deformation", Society of Automotive Engineers Transactions, 88(3) (1979) pp. 2630-2334, Paper No. 790741. The grid circle analyzer (GCA) system described by Ayres et al used photogridded solid circles on the sheet metal specimen, and all of the data processing was done in software by a digital computer.
Ayres et al concluded that the GCA system measured strain in sheet metal with accuracy that was comparable to optical techniques, in about one-third the time, and with greater objectivity than manual methods. They further concluded that the accuracy of the GCA system was dependent on both the sharpness of the edge and the contrast of the deformed circle to the background, that the GCA system could read strains in a large stamping without sectioning; and that the GCA system could be used by a relatively untrained operator.
The grid circle analyzer (GCA) system described by Ayres et al was further described in U.S. Pat. No. 4,288,852 by Steven W. Holland. The system converted the analog signals from the video camera to digital gray scale pixel data which was stored in the digital computer. The GCA system then required the computer to perform a time-consuming edge detection routine. This routine found the edge of the deformed solid circle by measuring the light intensity gradient at many locations in the digitized image. The gradient data was used to determine a threshold level for conversion of the digital gray scale image to a binary video image. The points residing on the edge of the deformed circle were then determined from this binary video image. An ellipse was fitted to the edge points, the major and minor diameters of the fitted ellipse were determined, and the strain induced in the sample was calculated by the computer as a function of the calculated diameters and the diameter of the circles before deformation.
While the GCA system described in the Ayres et al article and in the Holland U.S. Pat. No. 4,288,852 provided advantages over manual measurement systems for circle grid analysis, there has been a continuing need for improved systems which are easier and more convenient to use by an operator, which require less time to complete a strain analysis, and which require less computer storage and computation power.